feat(viewer): second Vite entry, viewer capability, CodeMirror deps

Task 6 of the terminal file viewer plan: app/viewer.html plus
src/viewer/{main,ViewerApp}.tsx (placeholder ViewerApp for Task 11 to
replace), registers viewer.html as a second Rollup input in
vite.config.ts, adds the file-viewer capability restricted to
file-viewer-* windows (allow-listen/unlisten for the goto event,
allow-destroy for the close-button/prevent_close interaction,
allow-internal-toggle-devtools to match default.json's dev
convenience), installs the CodeMirror packages Task 10 builds
languages.ts on top of (P1), and pins the viewer entry/capability
with a Rust fallback-trap test.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src-tauri/src/file_viewer/mod.rs

@@ -33,4 +33,19 @@ mod tests {
         assert!(!is_viewer_label("main"));
         assert!(!is_viewer_label("browser-view-abc"));
     }
+
+    /// Both Vite's dev server and Tauri's asset lookup fall back to `index.html`
+    /// when `viewer.html` is missing, so a broken entry opens the *main app* in
+    /// the viewer window with no error anywhere. Pin the two files the entry needs.
+    #[test]
+    fn the_viewer_entry_exists_and_is_a_vite_input() {
+        let app_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("..");
+        let html = std::fs::read_to_string(app_dir.join("viewer.html")).expect("app/viewer.html");
+        assert!(html.contains("/src/viewer/main.tsx"));
+        assert!(!html.contains("<style"), "an inline <style> makes Tauri add a style nonce, which disables 'unsafe-inline' and breaks CodeMirror");
+        let vite = std::fs::read_to_string(app_dir.join("vite.config.ts")).expect("vite.config.ts");
+        assert!(vite.contains("viewer.html"), "vite.config.ts must list viewer.html in build.rollupOptions.input");
+        let cap = std::fs::read_to_string(app_dir.join("src-tauri/capabilities/file-viewer.json")).expect("capability");
+        assert!(cap.contains("\"file-viewer-*\"") && cap.contains("core:window:allow-destroy"));
+    }
 }